This invention is concerned with the removal of residual organic matter from purified wet process phosphoric acid.
The Maurer et al U.S. Pat. No. 4,279,878, the entire disclosure of which is incorporated herein by reference, suggests that hydrogen peroxide has been used to attack chromophoric groups of organic impurities in phosphoric acid. However, this particular treatment was recognized as being insufficient to substantially remove organic impurities by oxidation of the organic impurities. (Note column 1, lines 29-44 and column 2, lines 52-63 of this Maurer et al patent.) Accordingly, the Maurer et al patent proposes a new process whereby phosphoric acid is heated in the presence of hydrogen peroxide and a catalyst to effect the oxidation of organic impurities. The use of a catalyst is essential, particularly in view of Example 3 (Comparative Example) on column 3, lines 26-32 of this Maurer et al patent. More particularly, this Comparative Example indicates that phosphoric acid, having a P.sub.2 O.sub.5 content of 60% (i.e. more than about 82% H.sub.3 PO.sub.4) which contains 440 ppm of organic carbon, still contains 290 ppm organic carbon after treatment with hydrogen peroxide in the absence of catalyst at a temperature of 80.degree. C. for 1 hour. Even when a catalyst is used, 110 ppm is the smallest quantity of organic carbon detected after treatment with hydrogen peroxide in the Examples of the Maurer et al patent. It is noted that the organic contaminants oxidized by the process of the Maurer et al patent apparently include residual solvent. (Note column 3, lines 2-14 of the Maurer et al patent.)
The Kikuchi et al U.S. Pat. No. 4,044,108, the entire disclosure of which is incorporated herein by reference, describes a process for the removal of both coloring organic impurities and non-coloring organic impurities from phosphoric acid by an oxidation treatment. These non-coloring organic impurities are described as materials which become colored impurities when the phosphoric acid is heated to temperatures of greater than 200.degree. C. for a sufficient time. (Note column 2, lines 17-30 of this Kikuchi et al patent.)
The process of the Kikuchi et al U.S. Pat. No. 4,044,108 involves the use of a very particular oxidizing agent, i.e. a chlorate, and a very particular phosphoric acid, i.e. greater than 90 wt. % H.sub.3 PO.sub.4. Thus, according to Example 2 on column 7, lines 1-23 of this Kikuchi et al patent, 90.9% H.sub.3 PO.sub.4 containing 590 ppm of carbon impurities was heated with 0.5 g of 35% HCl and 1.5 g of sodium chlorate at 110.degree. C. for 1 hour to obtain a clear colorless phosphoric acid with only a trace amount of organic material. When this colorless phosphoric acid was heated to 280.degree. C., no coloring was observed. However, when this same phosphoric acid was diluted with water to a concentration of 85% H.sub.3 PO.sub.4 and subjected to the same treatment as Example 2, according to Reference Example 7 on column 7, lines 31-40 of this Kikuchi et al patent, the treated phosphoric acid contained 230 ppm of carbon and produced noticeable coloring upon heating to 280.degree. C. Similarly, when 5.5 g of 30% H.sub.2 O.sub.2 replaced the 1.5 g of sodium chlorate in the procedure of Example 2, according to Reference Example 8 on column 7, lines 41-54, noticeable coloring was also observed when this hydrogen peroxide treated phosphoric acid was heated to 280.degree. C. Similar results with water dilution and hydrogen peroxide treatment, respectively, were obtained in Reference Examples 16 and 17 column 9, lines 13-38, of this Kikuchi et al patent.
It is noted that the particular phosphoric acid, which was subjected to treatment with an oxidizing agent in Example 2 and Reference Examples 8 and 17 of the Kikuchi patent, apparently contained organic impurities in the form of residual solvent. This conclusion is apparent from the fact that the 75.3% H.sub.3 PO.sub.4 was contacted with the non-volatile solvent, tributyl phosphoric acid, and then was concentrated to 90.9% H.sub.3 PO.sub.4 with total amount of carbon impurities increasing from 310 ppm to 590 ppm in the process.